They enforce this at the ROM level but not the SIM level, so any user-flashed ROM gets around this incompetent "restriction".

Same with Verizon

I'm positive I don't pay an additional charge for tethering on Verizon.

You might not given that provider plans and bundles do change over time. Sometimes there are still restrictions and drawbacks (e.g. throttling) since providers are opaque about their services. There could be new plans that include tethering but cost more or have bandwidth caps.

That already violates net neutrality.

Net neutrality rules never applied to wireless connections.

The principle applies to all traffic. Current rules are far from perfect on their own and have loopholes already. Instead of repealing them, they should be extended, including outright banning anti-competitive data caps which is a pure rip off.

Were I live packet inspection is illegal, regardless if its wired or mobile.

Yes they do.